(a) Field of the Invention
The present invention relates to a gain-controlled amplifier whose gain varies exponentially in response to a linear voltage variation of a control voltage applied to the amplifier.
(b) Description of the Prior Art
FIG. 1 shows one conventional gain-controlled amplifier 100. The amplifier 100 comprises an input terminal T1 to which an input signal Vi is applied, an output terminal T2 for outputting an output signal Vo, and a control-voltage terminal T3 to which a control voltage Vc is applied. The amplifier 100 further comprises operational amplifiers 101 and 102, current-mirror circuits 103 and 104, pairs of NPN transistors 105, 106 and 107 emitters of each of which are connected to each other. A gain Av of the amplifier 100 is expressed by the following equation when the resistance value of the input resistor 108 is R1 and when the resistance value of the feed back resistor 109 is R2: EQU Av=(R2/R1).multidot.exp (-K.multidot.Vc) (1)
where K=q/kT, and wherein k is Boltzmann's constant, q is an electric charge of an electron, and T the junction temperature represented in absolute temperature.
With the structure of the above-described conventional gain-controlled amplifier 100, however, a balanced output comprising two output signals identical in characteristic and 180.degree. out of phase from each other can not be obtained. For example, the amplifier 100 only outputs an output signal which is in phase with the input signal. And therefore, if an inverted output signal is necessary, the output terminal T2 of the amplifier 100 must be connected to an input terminal of an inverting amplifier 110 as shown in FIG. 2.
However, in such configuration as shown in FIG. 2, the degration of the output signal by the operational-amplifier 102 and that of the output signal by the inverting amplifier 110 differ from each other.